The present invention relates generally to ultrasonic distance measuring apparatus of the type which provides a distance output signal related to the time required for an ultrasonic wave to travel from a transmitter to a receiver and, more particularly, to such distance measuring apparatus in which reliability is enhanced and the possibility of erroneous distance measurement is reduced.
Various circuit arrangements have been utilized in the past to determine the distance between a transmitter and a receiver of ultrasonic acoustic wave energy. If the speed of sound in the medium through which the ultrasonic acoustic wave passes is known, the time period required for an acoustic wave to travel between the transmitter and receiver is proportional to the distance therebetween. Such a system has been utilized in numerous applications including, for example, measurement of the length or thickness of a pipe or other metallic element. In many such systems, the transmitter and receiver are positioned adjacent each other with the transmitted ultrasonic wave passing through the material and being reflected from the opposite surface of the material. In such an arrangement the transit time for the ultrasonic wave energy is twice the thickness or length of the material being measured.
One prior art device for measuring the time differential between transmitted and received ultrasonic acoustic energy waves is shown in U.S. Pat. No. 3,792,613, issued Feb. 19, 1974, to Couture. The Couture circuit provides for analog display of the transmitted and received ultrasonic pulses, with the time period between successive pulses being determined by their spacing on the CRT display.
As may be appreciated, any such ultrasonic distance measuring arrangement is subject to extraneous electrical noise. Additionally, if the system is operated in an acoustically noisy environment, such as a manufacturing plant, equipment in the plant may occasionally generate ultrasonic acoustic energy of a frequency detectable by the ultrasonic receiving transducer, resulting in the possibility that an inaccurate distance may be made. In order to reduce the occurrence of measurement errors, a number of prior art ultrasonic distance measuring circuits have utilized a range gate in the receiver circuitry which is enabled to pass a detection signal only at the estimated time of arrival of the received ultrasonic acoustic energy. U.S. Pat. No. 3,808,879, issued May 7, 1974, to Rogers; U.S. Pat. No. 4,014,208, issued Mar. 29, 1977, to Moore et al; and U.S. Pat. No. 3,929,006, issued Dec. 30, 1975, to Boggs et al, all show such ranging circuitry. In each of these circuits, a timer begins timing when an ultrasonic acoustic pulse is transmitted and opens a range gate at the approximate time subsequent to transmission at which it is expected to receive the transmitted ultrasonic acoustic pulse.
U.S. Pat. No. 3,554,013, issued Jan. 12, 1971, to Berg, shows a pulse-echo ultrasonic thickness measuring arrangement in which a range gate is opened at a fixed time subsequent to the transmission of the ultrasonic acoustic pulse. This time period is selected such that it corresponds to the time required for the pulse to traverse part of the transmitter structure, prior to entering the test material. Simultaneously with the opening of the range gate, a flip flop is set, initiating operation of a ramp generator. A pulse receiver detects reflection of the transmitted ultrasonic acoustic pulse and resets the flip flop via the range gate, thus terminating operation of the ramp generator. A peak detector circuit is provided for detecting the voltage level reached by the ramp output signal of the ramp generator, thus providing an analog indication of the time period between transmission and receipt of the ultrasonic wave energy.
All of the above range gating circuits provide for enablement of the range gate at a predetermined time period following transmission of a pulse of a ultrasonic energy. While this technique is acceptable when a distance of generally known magnitude is being measured, such an approach is not viable where the distance is unknown or where the distance may vary widely during operation of the system.
An additional problem exists with distance measuring apparatus of the type discussed above in which a sample and hold output circuit is provided with the output being updated during each transmit and receive operation. If, for some reason, the receiving transducer malfunctions, or the transmitting transducer ceases operation, the output will not be updated. If the measured distance thereafter changes, the output provided by the circuit then will not correspond to the altered distance. It is desirable, therefore, that the distance measuring circuit include an error detection arrangement for detecting malfunction in the operation of the system and providing an error indication output signal when such a malfunction occurs.